Converting a masked 2d grayscale image to an object
I used your image as height map and got this result:

Work in progress:



Take a plane and scale it up, subdivide it ~1000 times.
Add a displacement modifier with the image as texture.
To smooth out the result, add a subdivision surface modifier before and after the displacement modifier.
Apply the modifiers.
Go into edit mode (be careful with your modifiers, the first time Blender crashed at this step due to too many polygons) and select all the mesh. Go to "smooth vertex" and smooth it a lot (5 times with factor 1 on the screenshot; should have been more)
Add a decimate modifier to remove unnecessary geometry. Remove vertices based on the angle with the "planar" mode. I had to first decimate it with "collapse" because planar decimation takes a lot more of time to calculate. Ratio 0.1 and 6° angle limit worked well.
Go into edit mode again, select all vertices on the bottom with height 0 and delete them.